In the natural world, there are known various materials having a stacked layer structure (in the present invention, such a material having a stacked layer structure is called “a layered material”), such as mica and graphite as representative examples.
Known examples of this layered material may include, e.g., various layered silicates. In particular, layered silicate clay minerals such as montmorillonite, beidellite, saponite, hectorite and fluorotetrasilicon mica are well known. In these silicate materials, a silica tetrahedron layer and an Mg(OH)2 or Al(OH)3 octahedron layer are connected to form a constituent unit. In the case of a clay mineral, this tetrahedron or octahedron layer is negatively charged by the isomorphous substitution of a low valence ion. The positive charge corresponding to this negative charge is held between layers by the cation having ion exchangeability.
It is long known that various polar molecules are taken in by intercalation between layers to greatly change the interlayer distance and by the modification of crosslinking of the layers of layered silicate with alumina or the like, the stability can be enhanced or a large amount of polar molecules of various types can be adsorbed therein.
On the other hand, a zeolite material called MCM-22 is recently attracting attention as a highly active aluminosilicate catalyst. As described in Zeolite no Kagaku to Kogaku (Science and Engineering of Zeolite) (Non-Patent Document 1), a patent application for a method of synthesizing this material was filed by Mobil in 1990 (JP-A (Japanese Unexamined Patent Publication; KOKAI)-63-297210, Patent Document 1) and thereafter, Leonowicz et al. reported that this is a hexagonal zeolite having a peculiar pore structure. A representative material thereof is borosilicate having the following unit cell composition:H2.4Na3.1[Al0.4B5.1Si66.5O144]
The characteristic feature in the framework is to have two pore networks independent of each other in the direction perpendicular to the c axis (in the plane direction of layer). Among these pore networks, one is present between layers and a cocoon-like supercage (0.71×0.71×1.82 nm) is two-dimensionally connected to six supercages therearound. The supercages are directly connected to each other by a 10-membered ring and therefore, a relatively large molecule can enter into the pore as compared with a tunnel-like 10-membered ring pore. Another pore network is present within a layer and a two-dimensional network is formed by 10-membered ring zigzagged pores. ITQ-1 which is pure silica, SSZ-25 and the like have the same framework. IZA (International Zeolite Associate) recommends calling this structure by Structure Code MWW. Details on the structure are described, for example, in Atlas, 5th ed. or can be read on the internet, the homepage of IZA Structure Commission (http://www.iza-structure.org/) (as of January, 2003). The zeolite material having Structure Code MWW can be identified by its characteristic pattern of the X-ray diffraction (hereinafter simply referred to as “XRD”). As for the XRD pattern, for example, a simulation pattern of ITQ-1 can be available on the above-described homepage.
As a distinctive feature, this zeolite material is sometimes synthesized through a layered precursor (generally called MCM-22(P)). In the general production process therefor, the precursor can be obtained by a hydrothermal synthesis at 150° C. by using a relatively inexpensive hexamethyleneimine as the template. In the case of aluminosilicate, the precursor can be synthesized at an Si/Al molar ratio of 15 to 35. Unlike the production behavior of other zeolites, the material obtained by the hydrothermal synthesis is generally a layered precursor and when the precursor is calcined, dehydration condensation takes place between layers and MCM-22 having a zeolite structure is formed.
The MWW structure has a characteristic feature which has not seen in conventional zeolites as described above, and the aluminosilicate having the MWW structure is known to exhibit high activity and selectivity in the synthesis of ethylbenzene or cumene, as compared with those of zeolite having other structures or catalysts other than zeolite. Accordingly, it is considered that he aluminosilicate having the MWW structure is already used in many plants over the world.
Also, there is an attempt to obtain a catalyst having higher performance by utilizing the layered precursor which has been obtained in the synthesis of MWW structure. More specifically, MCM-36 obtained by crosslinking the layered precursor with silica (see, for example, W. J. Roth et al., Stud. Surf. Sci. Catal., 94, 301 (1995), Non-Patent Document 2), thin layered zeolite ITQ-2 obtained by the delamination (see, for example, A. Corma et al., Microporous Mesoporous Mater., 38, 301 (2000), Non-Patent Document 3) and the like have been reported and it is stated that these exhibit higher activity than aluminosilicate having a mere zeolitic MWW structure.
In the case of aluminosilicate, a process for producing a modified layered material having a structure analogous to MWW, other than the zeolite material (MWW structure) having a three-dimensional regular structure, by controlling the manner of the stacking between layers is established to a certain extent. This process is characterized in that, for example, MCM-22(P) as a layered aluminosilicate precursor is treated in an aqueous solution containing a surfactant such as hexadecyltrimethylammonium bromide to intercalate the surfactant between layers and thereby cause swelling and thereafter, the layers are crosslinked by silicate species to obtain a crosslinked layered material (MCM-36) or a layer is delaminated by ultrasonic wave irradiation or the like to form a so-called card house structure where the layers are joined with each other not only by plane-to-plane association but also by plane-to-edge association (ITQ-2). In either case, fundamentally, a process established for the modification of a layered silicate clay mineral is applied to MCM-22(P).
The MWW structure and the structure analogous thereto have a characteristic feature which has not seen in other zeolite structures as described above and therefore, a characteristic catalytic activity or adsorbing activity attributable to the structure can be expected. This characteristic activity is not necessarily limited to the above-described aluminosilicate but metallosilicate containing an element other than aluminum in the framework can be also expected to provide the same effect. From this expectation, various studies have been made on the synthesis of metallosilicate having an MWW structure or a structure analogous thereto. However, a transition element represented by titanium, vanadium and chromium, and a typical element of the 5th or greater period represented by indium and tin, which are expected to show remarkably different properties from aluminosilicate in general (not limited to MWW structure), have a very large ionic radius as compared with silicon or aluminum and therefore, such an element is difficult to introduce into the framework in many cases. Accordingly, a desired metallosilicate or a precursor thereof cannot be obtained in many cases by an easy and direct method of synthesizing, for example, allowing a compound containing such an element to be co-present in the raw material for the synthesis of zeolite.
For the purpose of introducing the element into the framework, various methods have been proposed. Representative examples of the method to be employed for the MWW structure may include a post-synthesis method (a method of once synthesizing zeolite and after-treating it to introduce a heteroelement into the framework; this is generally called a post-synthesis in contract with the direct synthesis) and an improved direct method.
With respect to the post-synthesis method, for example, U.S. Pat. No. 6,114,551 (Patent Document 2) discloses a process for synthesizing metallosilicate by a post-synthesis method, where aluminosilicate having an MWW structure is once synthesized, the whole or a part of aluminum is removed out of the system by a dealuminating treatment such as contact with SiCl4 in gas phase to form defects in the aluminosilicate, and a compound containing an element to be introduced thereinto, such as TiCl4, is contacted with the dealuminated product.
As for the improved direct method, Wu et al. have reported a method where ferrisilicate is obtained by designing the step of adding an iron compound to a gel (see, P. Wu et al., Chem. Commun., 663 (1997), Non-Patent Document 4).
Furthermore, for Ti which is difficult to introduce into the framework, a synthesis method using boron as a structure supporting agent has been recently developed (see, P. Wu et al., Chemistry Letters, 774 (2000), Non-Patent Document 5).
Also, a process for obtaining MWW-type titanosilicate has been proposed, where a large amount of boron is added to a starting raw material, an MWW precursor MCM-22(P) having both boron and titanium in the framework is synthesized by utilizing the function of boron as a structure supporting agent and after, if desired, removing boron by an acid treatment, the obtained precursor is calcined. The titanosilicate having an MWW structure prepared by this method is reported to exert a characteristic catalytic activity (see, P. Wu et al., J. Phys. Chem. B, 105, 2897 (2001), Non-Patent Document 6).
However, according to these methods, many elements which have been intended to be introduced thereinto cannot actually be introduced into the framework but remain as a residue in the pore. In the conventional post-synthesis methods of introducing a metal into zeolite, one important point for elevating the introduction efficiency is to select a compound which can easily enter the pores of zeolite. However, this can encounter a problem in some cases, for example, when a compound containing an element intended to be introduced and having a sufficiently small molecular size is not commercially available.
Furthermore, when the resultant product is used as a catalyst or the like, in a case where the raw material is a dealuminated MWW-type aluminosilicate as in U.S. Pat. No. 6,114,551 (Patent Document 2), a side reaction attributable to the aluminum remaining in the framework sometimes brings about a serious problem. The same problem occurs in the direct method using boron as a structure supporting agent. That is, boron cannot be satisfactorily removed even by an acid treatment and a large amount of boron remains in the framework or pores, or if strict conditions are set for the process of removing boron by an acid treatment or the like so as to enhance the efficiency of boron removal, elements which should remain in the framework are also disadvantageously removed at the same time. Moreover, the proper synthesis conditions are greatly affected by the element intended to be introduced and the compound containing the element and therefore, these methods are not good in view of the general-purpose applicability.
With respect to the process for producing metallosilicate having an MWW-analogous structure and not having a three-dimensional regular zeolite structure, where a transition element represented by titanium, vanadium chromium and iron or a typical element of the 5th or greater period represented by indium and tin is introduced into the framework, there has been reported by Corma et al. (see, Chem. Commun., 779-780 (1999), Non-Patent Document 7) a method of grafting a titanocene compound (TiCp2Cl2) to silica-type ITQ-2 which has bee prepared by the delamination and then calcining the resultant product.
However, the production process for silica-type ITQ-2 is not described in detail and the possibility of Al remaining cannot be denied. Furthermore, a decrease in the selectivity is described when Ti content is increased. Thus, this is not necessarily effective as a process for effectively introducing a metal such as titanium into the framework.
[Patent Document 1]
JP-A-63-297210
[Patent Document 2]
U.S. Pat. No. 6,114,551
[Non-Patent Document 1]
Zeolite no Kagaku to Kogyo (Science and Engineering of Zeolite), Kodansha, Jul. 10, 2000
[Non-Patent Document 2]
W. J. Roth et al., Stud. Surf. Sci. Catal., 94, 301 (1995)
[Non-Patent Document 3]
A. Corma et al., Microporous Mesoporous Mater., 38, 301 (2000)
[Non-Patent Document 4]
P. Wu et al., Chem. Commun., 663 (1997)
[Non-Patent Document 5]
P. Wu et al., Chemistry Letters, 774 (2000)
[Non-Patent Document 6]
P. Wu et al., J. Phys. Chem. B, 105, 2897 (2001)
[Non-Patent Document 7]
Chem. Commun., 779-780 (1999)